WASHINGTON — Immigration has supplanted Iraq as the leading issue on television and radio talk shows, complicating the prospects of a Senate bill desperately wanted by President Bush.

Conservative talk radio’s impact on the immigration debate reached new heights last week, with one host effectively writing an amendment for when the Senate returns to the imperiled bill this week.

National talk-show hosts have spent months denouncing the bill as providing amnesty for illegal immigrants. Some top Republicans who support the legislation have defied the broadcast pundits. Other GOP lawmakers have tried to placate them, even to the point of accepting their ideas for amendments.

Sen. Jon Kyl, R-Ariz., the key conservative negotiator behind the compromise bill, said Friday that California-based radio host Hugh Hewitt “had several ideas” that “we are trying to include” in amendments to be offered in an upcoming series of crucial votes.

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Hewitt, a conservative who has criticized many aspects of the bill, had Kyl as a guest Thursday and asked: “Does the bill provide for any separate treatment of aliens, illegal aliens from countries of special concern?”

Kyl replied: “It’s going to, as a result of your lobbying efforts to me.”

People seeking entry to the United States from countries designated as state sponsors of terrorism will receive a higher level of scrutiny, Kyl said Friday.

In a column posted on his Web site, Hewitt called Kyl “perhaps the single most effective and principled conservative in the United States Senate.”

The immigration bill would tighten borders and workplace enforcement, create a guest-worker program and provide ways to gain legal status for many of the estimated 12 million illegal immigrants in the United States.

The legislation faces showdown votes this week that lawmakers on all sides agree will be close.

If the measure fails, talk radio and TV — where CNN’s Lou Dobbs has been especially critical — will deserve substantial credit, academics and politicians say.

“Talk radio and talk TV are most effective when there’s an immediate action pending,” said Kathleen Hall Jamieson of the University of Pennsylvania, who is an authority on media and politics. “It’s a classic instance of mobilization with all the pieces in place, and it’s sure to have an effect.”

Sen. Jeff Sessions, R-Ala., a leading opponent of the bill, said that “talk radio has had a significant impact on this issue.”

A frequent guest of Dobbs, Hewitt and other conservative hosts discussing immigration, Sessions said, “I think people have learned more from talk radio than from reading the newspapers.”

Michael Harrison, editor of the talk-show industry magazine Talkers, said immigration has replaced the Iraq war as the most-discussed topic and has led many conservative hosts to show more loyalty to the anti-amnesty issue than to the Republican Party.

“I think talk radio should be credited with possibly saving the American people from George Bush’s immigration bill,” Harrison said, adding that he and his magazine are nonpartisan.

Some Republicans who recently announced their opposition to the bill said constituent concerns were their main reason. But they acknowledged the intensity of talk-radio hostility in their states.

“Neal Boortz, he popped us pretty good,” said Lindsay Mabry, a spokeswoman for Sen. Saxby Chambliss, R-Ga., who shifted from qualified support to opposition to the bill in recent days. She said Chambliss consulted with Boortz on immigration even though the senator was not an on-air guest during the debate.